I don't have Facebook account, so I cannot argue with your statement, but Geni is answering quite different issue than Facebook. Geni is about family trees.
On Geni you can have info about your ancestors while Facebook is for living friends and family.
Using both will give you much more than Facebook only.
Having been a Geni user for 3 years, I like to explain Geni to people as "Facebook for families". It's easier to focus around the greater family-unit BOTH on the present and on the past. Sharing current events and pictures ONLY with family is easier on Geni.
As a long-time Facebook user, its biggest failure is how "transient" everything is. Even if you made an effort to write-up something meaningful, and post a note about it, it will soon disappear forever, because on Facebook nothing lasts (this encourages Facebook to be very SHALLOW). This means that on Facebook there can be no past and NO future, only the NOW.
Geni enables us to recreate the past, so that both the present and the future have something to look for.
I had the same experience on Facebook...I had entered my family as far back as I could go....but just the main branch. When I would go back to look at what I enered...there would be people had disappeared. This happened numerous times. Eventually I just deleted everything. It was more of a bother than anything else.
Although I have a Facebook account, I don't use it all that much. I view my "work" on Geni (adding and updating profiles) to be meaningful. Updating part of a family tree for others in Geni, to me, actually feels like time spent worthwhile.
With Facebook posting I am not getting that feeling. I should try to utilize Facebook more effectively, but whenever I intend to go on there for 15 minutes it ends up being an hour "lost".
Twitter is fun, and something I could learn to use better, but it seems to have similar issues, even worse.
I am really puzzled about how to balance or properly make use of Geni, Facebook, and Twitter, and then decide what I just want to keep private, i.e. stuff you wouldn't know unless you sat down to have a cup of coffee with me face to face. Then there is the balance between stuff I would say, stuff I would write in a private diary, and stuff I would prefer to go to the grave with.
I have not found a balance yet, but one thing I do feel - if I spend an hour of free time updating a profile of an ancestor who lived 300 years ago, I think it is time well spent, and of some value to others related to that person, though they may not be aware of it yet.